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| United States Patent |
5,151,248
|
|
Ebara
, et al.
|
September 29, 1992
|
Pd-added austenitic stainless steel for use for high temperature
concentrated sulfuric acid
Abstract
An austenitic stainless steel for use for high temperature concentrated
sulfuric acid which comprises, on weight basis, 0.04% or less of C, 5-7%
of Si, 2% or less of Mn, 15-25% of Cr, 4-24% of Ni, 0.01-1.07% of Pd and
the rest consisting of Fe and unavoidable contaminant materials. By the
incorporation of small amount of palladium in a basal austenitic stainless
steel containing the essential three elements of Cr, Ni and Si, a superior
corrosion resistance against highly concentrated high temperature sulfuric
acid is attained.
| Inventors:
|
Ebara; Ryuichiro (Hiroshima, JP);
Nakamoto; Hideo (Hiroshima, JP);
Ukawa; Naohiko (Hiroshima, JP);
Yamada; Tamotsu (Hiroshima, JP);
Nishimura; Yasuo (Tokyo, JP)
|
| Assignee:
|
Mitsubishi Jukogyo Kabushiki Kaisha (Tokyo, JP)
|
| Appl. No.:
|
807633 |
| Filed:
|
December 13, 1991 |
Foreign Application Priority Data
| Current U.S. Class: |
420/35; 420/50; 420/584.1 |
| Intern'l Class: |
C22C 038/40 |
| Field of Search: |
420/35,50,584
|
References Cited
U.S. Patent Documents
| 4018569 | Apr., 1977 | Chang | 428/678.
|
| 4384891 | May., 1983 | Barnabe 420 35.
| |
| Foreign Patent Documents |
| 2079787 | Jan., 1982 | GB.
| |
Other References
Davies et al., Application of Saramet in H.sub.2 SO.sub.4 Plants, British
Sulphur's 13th International Conference, Nov. 1988.
|
Primary Examiner: Yee; Deborah
Attorney, Agent or Firm: Fleit, Jacobson, Cohn, Price, Holman & Stern
Parent Case Text
This is a continuation of application Ser. No. 07/700,437, filed May 15,
1991, which was abandoned upon the filing hereof.
Claims
We claim:
1. An austenitic stainless steel for use for high temperature concentrated
sulfuric acid, comprising, on weight basis, 0.04% or less of C, 5-7% of
Si, 2% or less of Mn, 15-25% of Cr, 4-24% of Ni, 0.01-1.07% of Pd and the
rest consisting of Fe and unavoidable contaminant materials.
2. An austenitic stainless steel as claimed in claim 1, wherein the
unavoidable contaminant materials contain P and S each in a minute amount.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an austenitic stainless steel superior not
only in the workability but also in the corrosion resistance for use for
the material of, such as, absorption towers, cooling towers, pumps,
vessels and so on, to be employed in an environment of high temperature
concentrated sulfuric acid in sulfuric acid industry, in particular, for
dealing with sulfuric acid of a concentration of 90-100% at a temperature
of up to 240.degree. C.
2. Description of the Related Art
Sulfuric acid has in general a high corrosive property for metals. Such
attack of metals by sulfuric acid is quite considerable especially at
medium concentrations of sulfuric acid from about 10 to about 80%. This is
attributed mainly to the fact that such medium concentration sulfuric acid
is a non-oxidative acid. Existing materials capable of withstanding such
sulfuric acid environment are quite limited and may be exemplified, for
use at temperatures below 100.degree. C., by lead and some of Ni alloys,
such as, Hastelloy B and C276 (trade names).
It is known, on the other hand, that oxidizing conditions take place when
sulfuric acid is concentrated up to 90% or higher. For such highly
concentrated sulfuric acid, some metals which do not withstand a medium
concentration sulfuric acid may become tolerable for use. For example,
mild steel has a better corrosion resistance against a highly concentrated
sulfuric caid of 98% at lower temperatures, due to formation of an
anti-corrosive protective layer of FeSO.sub.4 over the entire surface of
the steel, so that it finds practical use for such highly concentrated
sulfuric acid at room temperature (at around 20.degree. C.).
At higher temperatures up to 240.degree. C. to be encountered in sulfuric
acid industry, the attacking action of sulfuric acid becomes violent. At
such a high temperature, the protective FeSO.sub.4 coating layer of milde
steel will tend to dissolve in the highly concentrated sulfuric acid to
destroy the anti-corrosive passive layer, resulting in destruction of
corrosion resistance of mild steel.
Usual austenitic steels, various ferrite steels and nickel alloys exhibit
poor corrosion resistance in such highly concentrated high temperature
sulfuric acid and even lead and Ni-alloys, such as Hastelloy B and C-276
(trade names), exihibiting relatively high corrosion resistance in medium
concentration sulfuric acid become less resistant at high temperatures to
highly concentrated sulfuric acid.
No material has been found up to date, which has sufficient resistance in
such environment and which is applicable practically for various
installations and instruments in the sulfuric acid industry. However, it
has been known, that high Si cast iron (containing more than 14% of Si)
exhibits relatively superior corrosion resistance in high concentration
sulfuric acid at lower temperatures (below about 120.degree. C.). It has
been assumed that Si contributes effectively to the development of
anti-corrosive property effectively. It has recently been reported that
ferritic stainless steels having high content of Cr exhibit also
relatively better corrosion resistance in such an environment. This
suggests that Cr may contribute to the development of corrosion resistance
effectively and that the content of Ni which is assumed to have a negative
effect on the development of anti-corrosive property is low.
However, these steels have poor mechanical workability and, in particular,
high Si cast iron is scarcely able to be subject to mechanical working and
welding, so that it finds no practical use for large sized installations
and instruments. Thus, in the practice, large sized installations to be
employed in an environment of highly concentrated sulfuric acid of above
90% at a temperature of up to 120.degree. C., such as, absorption towers
and so on, are lined internally with acid-resistant bricks.
Such internal lining suffers from the problems such as follows:
The binder material employed to fill up the interstices between the
adjoining acid-resistant bricks will be damaged during the course of
long-term operation by the highly concentrated sulfuric acid, which may
cause leakage of sulfuric acid, so that it is necessary to incorporate an
overhauling of the entire installation at intervals of a few years. Such a
damage of the binding material will markedly be accelerated under the
conditions with which the present invention deals, namely, sulfuric acid
of a concentration of above 90% and a temperature of up to 240.degree. C.
and the durability of the brick will also promotively be damaged.
Also, high Cr ferritic stainless steels which have relatively better
corrosion resistance as compared with other materials will suffer from
corrosion attack under the condition mentioned above and will be subject
to a corrosion rate exceeding over the critical allowable value of 0.1
g/cm.sup.2.hr for the practical use. This is because that the content of
Cr is not allowed to reach the amount necessary for attaining sufficient
corrosion resistance under the condition mentioned above, namely, over
35%, in order to maintain a tolerable workability. When the content of Cr
is increased, the resulting high Cr ferritic stainless steel becomes
brittle and mechanical working, such as, pressing and rolling, becomes
difficult. Upon welding such a high Cr ferritic stainless steel,
incorporation of additional technical measures, such as, preheating,
after-heating and so on, is necessary for avoiding the hardening of the
material around the welded portion, resulting in a considerable increase
in the costs for manufacturing and overhauling such installations, as
compared with materials of austenitic stainless steels.
As for high Si cast iron, the problem that a mechanical working and welding
will scarcely be permitted due to the brittleness of the high Si cast iron
is left unsolved.
Under the circumstances of the stand of the technique described above, it
is contemplated by the present invention to provide a novel austenitic
stainless steel which resolves the disadvantage of poor corrosion
resistance associated with the conventional material in the environment of
highly concentrated high temperature sulfuric acid and which permits
welding and mechanical working without problem.
SUMMARY OF THE INVENTION
Thus, the present invention provides an austenitic stainless steel
containing a small amount of palladium and exhibiting a markedly increased
corrosion resistance under the environment of highly concentrated high
temperature sulfuric acid, which comprises, on weight basis, 0.04% or less
of C, 5-7% of Si, 2% or less of Mn, 15-25% of Cr, 4-24% of Ni, 0.01-1.07%
of Pd and the rest consisting of Fe and unavoidable contaminant materials.
The essential characteristic feature of the austenitic steel according to
the present invention resides in that it comprises three basal elements of
Cr, Ni and Si with addition of a small but suitable amount of Pd for
attaining a considerably increased corrosion resistance under the
environment of highly concentrated high temperature sulfuric acid. In the
following, the functions and effects of each component element of the
alloy steel according to the present invention will be described with
reference to the appended drawings by way of concrete embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing the relationship between the Si content of steel
and the corrosion rate of the steel in highly concentrated high
temperature sulfuric acid.
FIG. 2 shows the comparison of temperature dependence of the corrosion rate
between the steel according to the present invention and conventional
steels.
FIG. 3 is a graph showing the relationship between the Pd content and the
corrosion rate for the steel according to the present invention.
FIG. 4 is a graph showing the comparison of corrosion resistance and
mechanical workability between the steel according to the present
invention and conventional steels.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The experimental results and the composition of the steel for each of
Examples 1 to 10 and Comparison Examples 11 to 22 are summarized in Table
1.
The each essential component element of the steel according to the present
invention has been selected based upon the knowledge and consideration
from the experiments as described below:
It has been known that high Si cast iron has a relatively better corrosion
resistance to highly concentrated (90-100%) sulfuric acid at higher
temperatures (100.degree.-120.degree. C.). This suggests that Si has a
certain effect on improving the corrosion resistance of a steel to such
sulfuric acid environment. It is also known that increase in the content
of Cr in a stainless steel will impart to the steel an improved corrosion
resistance to such sulfuric acid environment.
However, it is required in an austenitic stainless steel to increase the Ni
content in correspondence to an increase in the total content of the
ferrite-forming elements, namely, Cr+Si, in order to maintain the
austenite phase which provides a better mechanical workability. It is
necessary and preferred to limit the content of Ni in the stainless steel
according to the present invention to the minimum amount necessary for
maintaining the austenite phase, since it is known that content of Ni has
a negative effect on a stainless steel in attaining corrosion resistance
to the environment of highly concentrated high temperature sulfuric acid
to be dealt with by the present invention.
Supported by such knowledge, the inventors made an investigation into a
possibility of improving the corrosion resistance of an austenitic
stainless steel in such an environment of highly concentrated high
temperature sulfuric acid by an increased content of Si in a basal
austenitic stainless steel under preservation of the austenite phase
attributive to better weldability and higher workability of the steel, in
consideration of Schaeffler's phase diagram (a diagram showing the
relationship between the metal structure and equivalent proportion of each
component alloy element), whereby it was confirmed experimentally that
increased contents of Si in the austenitic basal alloy steel will bring
about an improvement in the corrosion resistance of the basal austenitic
steel against the environment of highly concentrated high temperature
sulfuric acid, as shown in FIG. 1.
It is seen in FIG. 1 that the anti-corrosive property of the basal
austenitic steel is improved remarkably by the content of Si in an amount
over 5%. However, an excessive content of Si in the steel brings about a
considerable increase in the hardness of the steel and, when the Si
content exceeds about 7%, the increase in the hardness goes beyond the
permissible limit for allowing rolling work. Thus, the upper limit of Si
content in an austenitic stainless steel for preserving permissible
workability may assumably be at about 7%.
While, as confirmed experimentally, a better anti-corrosive property is
imparted to an austenitic stainless steel by adding Si, the Si content may
preferably be lower enough to allow better mechanical working, such as,
rolling, pressing and so on. The inventors had therefore looked for a
measure for realizing lower possible content of Si in such a basal
austenitic stainless steel while maintaining sufficient mechanical
workability with enough corrosion resistance against said sulfuric acid
environment and have found that addition of small amount of palladium to
such basal austenitic stainless steel provides the practical solution
therefor. Thus, as shown in FIG. 2, it was discovered that an addition of
small amount of Pd to the basal austenitic stainless steel will bring
about a remarkable improvement in the anti-corrosive property of the basal
austenitic steel under the environment of highly concentrated high
temperature sulfuric acid.
According to a further study carried out by the inventors, it was
confirmed, as shown in FIG. 3, that, with a fixed Si content of 5.5%, the
maximum anti-corrosive effect was attained when the Pd content was in the
range from 0.2 to 0.6%. Furthermore, it was shown, as seen in Table 1,
that a better anti-corrosive property was attained at an Si content of
6.61%, even when the content of Pd amounted to only 0.01%.
While the essential features of the present invention have been given in
the paragraph of "Summary of the Invention" and the scope of the present
invention is defined in the Patent claims, such a definition of the
present invention has been based on the reasons described below.
.largecircle. As to the content of carbon (C)
While C has a negative effect on the anti-corrosive property of the basal
austenitic steel, it has a positive effect on the development of strength
of the steel and some content thereof should be present. Since the
anti-corrosive property deteriorates markedly when the carbon content
exceeds over 0.04%, the partinent content of C should be in the range from
0.004 to 0.04%.
.largecircle. As to the content of silicon (Si)
Si constitutes one of the essential elements of the basal austenitic
stainless steel of the present invention and has a fundamental
contribution to the development of not only the anti-corrosive property
but also the anti-oxidative nature of the steel. The anti-corrosive
property of the basal austenitic steel is improved remarkably by an Si
content of above 5%. An increase in the Si content also results in an
improvement in the anti-corrosive property. However, an Si content over 7%
may cause deterioration of mechanical workability. Therefore, the
partinent content of Si may be in the range from 5 to 7%.
.largecircle. As to the content of manganese (Mn)
Manganese serves as a deoxidizer and is employed in an amount below 2% of
the alloy from the point of view of anti-corrosive property of the steel.
In the Examples, it was incorporated in the steel in an amount in the
range from 0.49 to 0.60%.
.largecircle. As to the content of chromium (Cr)
Chromium constitutes one of the essential tertiary elements of the basal
austenitic stainless steel according to the present invention. It is
necessary, in general, to choose a content of chromium of at least 15%, in
order to attain a sufficient anti-corrosive property according to the
present invention under the environment of highly concentrated high
temperature sulfuric acid. While the anti-corrosive property of the steel
improves with increasing the content of chromium, a corresponding increase
in the content of Ni becomes necessary for maintaining the austenite phase
of the steel and such an increase may counteract to the development of
anti-corrosive property due to debasement of the corrosion resistance by
higher Ni content. When the content of Cr exceeds 25%, forging becomes
difficult. Thus, the pertinent content of Cr should be in the range from
15 to 25%.
.largecircle. As to the content of nickel (Ni)
Ni is necessary for maintaining the austenite phase and should be present
in an amount in the range from 4 to 24%.
.largecircle. As to the content of palladium (Pd)
Palladium constitutes one of the essential elements of the austenitic
stainless steel according to the present invention, though it is employed
in a small amount. It provides a remarkable improvement of the corrosion
resistance against the environment of highly concentrated high temperature
sulfuric acid. The effect of improvement of the corrosion resistance is
attainable at a Pd content of at least 0.01% and such effect increases as
the content of Pd becomes higher. However, a Pd content over 1.07% is
meaningless and uneconomical, since the effect of improvement of the
corrosion resistance reaches the saturation at this content. Thus, the
partinent content of Pd is in the range from 0.01 to 1.07%.
.largecircle. As to the unavoidable contaminant materials
They encompass phosphorus (P), sulfur (S), oxygen (0) and so on.
Phosphorus (P) should preferably be contained as little as possible in view
of the anti-corrosive property and of hot workability. If it exceeds
0.03%, the hot workability deteriorates.
Sulfur (S) has, like phosphorus, also a large effect on the mechanical
workability of the steel and should not be present in an amount higher
than 0.014%.
Oxygen should also be kept in the steel as little as possible for the
reason similar to that for P and S and the content thereof should
preferably be lower than 50 ppm.
It is preferable that the sum of the contents of S and 0 does not exceed
150 ppm.
Examples of the austenitic stainless steel according to the present
invention exhibiting a higher anti-corrosive property together with a
better mechanical workability comparable to those of conventional
anti-corrosive steels are summarized in Table 1 for the alloy composition
and the experimental data in comparison with those of conventional steels
(Comparison Examples).
The experimental data given in Table 1 are plotted in the graph of FIG. 4
for easy comparison between the steel according to the present invention
(indicated by closed circles) and the conventional steel (indicated by
open circles).
As a workability index used in FIG. 4, -R is defined as follows:
-R=-[(equivalent of Cr) minus (equivalent of Ni) ]
in which the equivalent of Cr is calculated by
Cr+Mo+1.5 Si
and the equivalent of Ni is calculated by
Ni+0.5 Mn
The value of R, namely, (eq. of Cr)-(eq. of Ni) is an index for the degree
of ease of mechanical working. In general, this value is greater for less
workable materials having higher Cr content (for example, the materials
SUS 447 J and EB26-1 as given in FIG. 4) and it falls in the range from 7
to 20 for materials exhibiting a relatively better workability and
supplied in the market in large amounts (for example, the materials SUS
316L, SUS 304L and so on as given in FIG. 4).
For the Comparison Examples, conventional steels widely produced with solid
production records are selected for comparison.
The values of R for Inconel 625 and C 276 are given only by numbers in the
graph of FIG. 4, since the values are too large and cannot be plotted on
the proper position in the graph.
[Experiments ]
The variation of the hot workability and the anti-corrosive property due to
the variation of the alloy composition was investigated for alloy steels
according to the present invention (Examples 1 to 10) and for alloy steels
of the stand of the technique (Comparison Examples 11 to 22). The alloy
steels according to the present invention were prepared in such a manner
that the metal components are melted in a vacuum arc smelting furnace and
the resulting metal ingot is subjected to a surface treatment before it is
hot rolled under a condition normally used for a stainless steel,
whereupon the resulting hot rolled strip is subjected to a solid solution
treatment. Each specimen of the alloy steels was examined by a corrosion
test in which the specimen was immersed in a 98% conc. sulfuric acid at a
temperature in the range of, in most cases, 100.degree.-220.degree. C. for
24 hours and the weight loss due to the corrosion was determined by
accurately weighing the specimen before and after the immersion.
For the workability of the steels, the values of the workability index
explained above were calculated only because such an index is convenient.
As explained above, the calculation was based on the equation:
-R=-[(equivalent of Cr) minus (equivalent of Ni)]
in which the equivalent of Cr is calculated by
Cr+Mo+1.5 Si
and the equivalent of Ni is calculated by
Ni+0.5 Mn
From the data given in Table 1, it is clear that the austenitic stainless
steels according to the present invention having a Pd content of 0.5%
(Examples 2, 3 and 4) are superior in the corrosion resistance against the
highly concentrated sulfuric acid as compared with the prior art steel
having a similar composition without Pd content (Comparison Example 17).
It is seen further that the corrosion resistance of the steels according
to the present invention having a Pd content of 0.5% (Examples 2, 3 and 4)
is superior than that of the steels according to the present invention
having a Pd content of 1.07% (Examples 5 and 6).
It is seen moreover, that the workability of the steels according to the
present invention may be comparable to that of the convectional steel for
use in the environment of sulfuric acid employed practically and most
frequently (Comparison Example 11).
As described in detail above, an austenitic stainless steel for use in an
environment of highly concentrated high temperature sulfuric acid which
exhibits superior anti-corrosive property together with better workability
and which is based upon a basal alloy steel containing the three elements
of chromium, nickel and silicon with addition of a small amount of
palladium can be provided by the present invention. The austenitic
stainless steel according to the present invention offers a wider
applicability in the sulfuric acid industry due to its superior corrosion
resistance even at higher temperatures together with its better
workability.
TABLE 1
______________________________________
Examples No.
1 2 3 4 5
______________________________________
Composition (%)
C 0.013 0.011 0.011 0.011 0.014
Si 5.21 5.63 5.63 5.63 5.41
Mn 0.60 0.52 0.52 0.52 0.55
P 0.012 0.013 0.013 0.013 0.013
S 0.011 0.011 0.011 0.011 0.010
Ni 4.02 17.72 17.72 17.72 17.49
Cr 17.62 17.65 17.65 17.65 17.58
Mo -- -- -- -- --
Cu -- -- -- -- --
Pd 0.10 0.51 0.51 0.51 1.07
N -- -- -- -- --
Others -- -- -- -- --
Denotation -- -- -- -- --
Workability
21.18 8.20 8.00 8.10 8.05
Index R.sup.1)
Corrosion test
Temp. (.degree.C.)
220 160 180 220 180
Corrosion Rate.sup.2)
0.17 0.03 0.05 0.18 0.13
______________________________________
Example No.
6 7 8 9 10
______________________________________
Composition (%)
C 0.014 0.011 0.013 0.016 0.015
Si 5.41 6.61 5.23 5.30 5.32
Mn 0.55 0.51 0.55 0.49 0.60
P 0.013 0.012 0.014 0.013 0.012
S 0.010 0.011 0.010 0.010 0.010
Ni 17.49 17.64 18.24 18.61 18.74
Cr 17.58 17.65 20.62 22.31 24.65
Mo -- -- -- -- --
Cu -- -- -- -- --
Pd 1.07 0.01 0.52 0.51 0.49
N -- -- -- -- --
Others -- -- -- -- --
Denotation -- -- -- -- --
Workability
8.07 9.72 9.99 11.46 13.68
Index R.sup.1)
Corrosion test
Temp. (.degree.C.)
220 180 180 180 180
Corrosion Rate.sup.2)
0.14 0.03 0.06 0.05 0.06
______________________________________
Comparison Example No.
11 12 13 14 15
______________________________________
Composition (%)
C 0.016 0.003 0.04 0.14 0.005
Si 0.67 0.03 0.16 0.014 0.09
Mn 1.27 0.50 0.27 0.88 0.11
P 0.031 0.010 0.004 0.028 0.013
S 0.003 0.005 0.001 0.001 0.003
Ni 12.07 Bal. 60.9 7.21 --
Cr 17.30 15.40 20.90 25.15 26.79
Mo 2.05 15.6 8.8 3.20 1.30
Cu -- -- -- 0.47 --
Pd -- -- -- -- --
N -- -- -- 0.14 0.08
Others -- .sup.3) .sup.4)
Denotation 316L C276 Inco- 329J.sub.2 L
EB26-1
nel625
Workability
7.65 -27.05 -31.1 21.32 28.18
Index R.sup.1)
Corrosion test
Temp. (.degree.C.)
180 180 180 180 180
Corrosion Rate.sup.2)
6.61 6.60 4.17 2.78 0.90
______________________________________
Comparison Example No.
16 17 18 19 20
______________________________________
Composition (%)
C 0.012 0.015 0.74 0.010 0.07
Si 4.03 5.52 14.85 0.60 0.75
Mn 0.55 0.50 0.38 1.22 0.79
P 0.015 0.019 0.05 0.033 0.014
S 0.010 0.010 0.01 0.005 0.001
Ni 17.55 17.62 -- 10.42 19.12
Cr 17.50 17.53 -- 18.24 25.06
Mo 0.031 0.054 -- -- --
Cu 0.020 0.029 0.43 -- --
Pd -- -- -- -- --
N 0.031 0.040 -- -- --
Others -- -- -- -- --
Denotation .sup.6) .sup.7) .sup.8)
SUS SUS
394L 310S
Workability
5.75 7.99 .sup.9)
8.11 6.67
Index R.sup.1)
Corrosion test
Temp. (.degree.C.)
180 180 180 -- --
Corrosion Rate.sup.2)
1.61 0.30 0.01 0.927 0.297
______________________________________
Comp. Example No.
21 22
______________________________________
Composition (%)
C 0.004 0.06
Si 0.11 0.25
Mn 0.11 0.50
P 0.020 0.032
S 0.005 0.005
Ni -- --
Cr 30.38 16.62
Mo -- --
Cu -- --
Pd -- --
N -- --
Others -- --
Denotation SUS 447J.sub.1
SUS 430
Workability 30.49 16.75
Index R.sup.1)
Corrosion test
Temp. (.degree.C.)
-- --
Corrosion Rate.sup.2)
0.581 1.700
______________________________________
.sup.1) R = (Cr + Mo + 1.5 Si) - (Ni + 0.5 Mn)
.sup.2) Of units in g/cm.sup.2 /hr
.sup.3) Co 0.80, W 3.70 and Fe 6.1
.sup.4) Ti 0.24, Al 0.29, Cd + Ta 3.51 and Fe 4.0
.sup.5) W 0.34
.sup.6) 17.5Cr--17.5Ni--4Si
.sup.7) 17.5Cr--17.5Ni--5.5Si
.sup.8) High Si cast iron
.sup.9) Impossible of rolling work
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